Remove code duplications

[KFilipek]

Avoid duplications in PMSE code.
Replace duplicated parts of code with functions.

[jschmieg]

Please be more specific in such a request. It is not possible to estimate without specifying which parts of code must be corrected.

[KFilipek]

I had in mind Pmse Tree, but this task applies to every class.

[jschmieg]

Please specify to which parts of code this Issue applies, i.e.:
"Replace allocation functionality (transaction::exec_tx...{ obj=pmemobj_tx_alloc...}) in PmseTree::redistributeNodes, PmseTree::coalesceNodes( and other functions) with dedicated method to allocate from memory and memcpy to it"